A flat monolithic accelerometer conventionally comprises a body having a base and two measurement cells providing a differential measurement. A measurement cell typically comprises a seismic mass connected, on one side, to the base and, on the other side, to a force sensor, which is itself also connected to the base. When the accelerometer is subjected to an acceleration along the sensitive axis, which is the axis of the acceleration to be measured, the seismic mass is subjected to an inertial force, which is amplified and transmitted to the force sensor by means for amplifying the force or displacement transmitted.
In patent application FR 0 102 573, the amplification is obtained by means of an arm called a lever arm, which extends the seismic mass. The displacement of the seismic mass is transmitted to the force sensor by means of this lever arm. More precisely, the arm is connected to the base via an articulation, allowing the mass to rotate about an axis perpendicular to the sensitive axis of the accelerometer, and is connected to the force sensor via a hinge. When the accelerometer is subjected to an acceleration along the sensitive axis, the seismic mass is subjected to a force which rotates it about the articulation, as therefore that part of the lever arm which is connected to the force sensor.
The force sensor is a vibrating-beam sensor. The vibrating beam is connected to electrodes that allow it to vibrate at its resonant frequency and to a circuit for measuring the variation in its resonant frequency.
The measurement cells are mounted so that when the accelerometer is subjected to an acceleration along the sensitive axis one of the beams is subjected to a tensile force, the other beam being subjected to a compressive force of the same value, these tensile or compressive forces varying the resonant frequency of the beam measured by the measurement circuit. Thus, a differential measurement is obtained, which makes it possible in particular to overcome certain nonlinear effects.
The variation in the resonant frequency is directly related to the displacement of the force sensor induced by the rotation of that part of the lever arm which is connected to the force sensor. The end of the beam also undergoes a certain rotation, which often proves to be problematic, especially in the case of a tuning fork (i.e. rotation of two beams that form a tuning fork) in which the force transmitted to the two beams is not exactly identical.
In addition, the quality of machining of the hinges and articulations is of paramount importance, and constitutes one of the industrial limitations of this accelerometer.
Furthermore, since the displacement is proportional to the length of the lever arm, the overall size is greater the higher the desired amplification ratio to be obtained.
An important object of the invention is therefore to propose an accelerometer that does not have a rotating lever arm as amplification means but comprises, in general, a resonator that may be a vibrating beam.